Guayule is a desert shrub native to the southwestern United States and northern Mexico and which produces polymeric isoprene essentially identical to that made by Hevea rubber trees (e.g., Hevea brasiliensis) in Southeast Asia. As recently as 1910 it was the source of half of the natural rubber used in the U.S. Since 1946, however, its use as a source of rubber has been all but abandoned in favor of cheaper Hevea rubber and synthetic rubbers. However, demand for natural rubber is expected to produce shortages of that material in the future and rubber prices are expected to rise significantly. Natural rubber having lower heat hysteresis is required for many kinds of tires and amounts to about 35% of U.S. rubber use.
As an alternative to synthetic rubber sources, attention is being directed to the production of hydrocarbons in plants such as guayule (Parthenium argentatum). Guayule normally yields one half ton to one ton of rubber per acre in cultivation when, after two years, the entire plant is harvested and processed. Guayule plants store latex in tiny inclusions in the bark, making harvest of the outer fibrous layers, or bagasse, of the plant, desirable.
Using traditional techniques, as much as 95% of the available natural rubber may be recovered from plant materials, using parboiling, which coagulates the latex in the cells, followed by a milling step in a caustic solution to release the rubber. This traditional process then causes the milled bagasse to sink to the bottom of the processing vessel and allows resin to float to the surface for collection. More specifically, in traditional processes, resins from plant materials are obtained by solvent extraction with polar solvents such as alcohols, ketones, and esters. A commonly used solvent for extracting the guayule resin is acetone. The resin is recovered from the solution by evaporating the solvent. The rubber from the shrub is generally extracted using hydrocarbon solvents such as hexane, cyclohexane or toluene. Such processes are normally very expensive and not environmentally friendly. A water floatation method has also been used for the extraction of rubber.
Further, using traditional methods of guayule processing, plant material is prepared by initially grinding it into small particles. Generally, the entire plant is fed whole, that is, with the leaves thereon as well as dirt or foreign debris, into a grinding apparatus, for example, a hammermill. The ground material can be flaked, that is crushed, by adding to a two-roll mill or other conventional equipment, which ruptures the rubber-containing cells. The communited plants are subjected to a resin-rubber solvent system. The solvent system contains one or more solvents which extract the resin as well as the rubber from the guayule-type shrub. Examples of single-solvent systems include halogenated hydrocarbons having from 1 to 6 carbon atoms, such as chloroform, perchloroethylene, chlorobenzene, and the like; and aromatic hydrocarbons and alkyl-substituted aromatic hydrocarbons having from 6 to 12 carbon atoms, such as benzene, toluene, xylene, and the like.
This solvent system typically contains one or more polar resin solvents as well as one or more hydrocarbon rubber solvents. Typical polar resin solvents include alcohols having from 1 to 8 carbon atoms, such as methanol, ethanol, isopropanol and the like; esters having from 3 to 8 carbon atoms such as the various formates, the various acetates and the like; ketones having from 3 to 8 carbon atoms, such as acetone, methyl ethyl ketone, and the like. Typical non-polar hydrocarbon rubber solvents include alkanes having from 4 to 10 carbon atoms, such as pentane, hexane, and the like; and cycloalkanes having from 5 to 15 carbon atoms, such as cyclohexane, decalin, the various monoterpenes, and the like. Although the two types of solvents can form a two-phase system, they often form a single phase when utilized in proper proportions. One manner of adding different type solvents to the shrub is separately, but simultaneously. However, they are generally prepared as a mixture and added as such.
Accordingly, numerous combinations of a polar resin solvent and a hydrocarbon rubber solvent can exist. A specific solvent system is an azeotropic composition of approximately 80% by weight of pentane, more specifically 78.1% by weight, and 20% by weight of acetone, more specifically 21.9% by weight. The ratio by weight of solvent to the amount of shredded shrub can be any amount sufficient to generally extract most of the rubber and resin, as for example from about 1 part by weight of solvent up to about 20 parts by weight of solvent for each 1 part by weight of shrub, and preferably about 3 parts by weight of solvent to 1 part by weight of shrub. The rubber-resin miscella so obtained typically contains about 1 to 25% by weight of total solids, that is resin plus rubber, and preferably about 9 to 18% by weight of total solids with the amount of resin by weight being from about 1 to about 3 parts for every 1 part by weight of rubber.
Furthermore, traditional methods of plant processing have been hampered by the use of these highly toxic compounds and cumbersome processes. For example, in prior industrial operations, hexane and heptane solvents have been used in the solvent extraction of oil-containing vegetable matter. The extraction apparatus typically includes vertical extraction towers, screw extractors and bucket extractors. With current equipment, several extraction stages are necessary in order to circulate the miscella and attain sufficient wetting of the material to be extracted, thereby requiring the use of a higher proportion of solvent.
In addition, overall energy consumption inherent in previous slurry separations has been excessive, if not prohibitive. Processing of this type of plant material traditionally requires wetting to form a liquid slurry, a high amount of heat, and a difficult separation of the solvent from the extracted oil and defatted meal. Complete removal of solvents, such as hexane, from the spent botanical residue is practically impossible by conventional steam stripping techniques.
The method of using gaseous solvents at both supercritical and subcritical conditions, such as carbon dioxide and propane, is also problematic. In these systems, the operating pressure must exceed 125 psi to remain in liquid state and even higher if temperatures are elevated. Because of the difficulties in working at high pressure, multiple extraction vessels are required, which limits the speed and efficiency of these extractions. Further, it is difficult to maintain pressures consistently, resulting in freezing, gumming, or poor separation of the extracted materials, which may clog the system. Also hydrolysis of lipids or inadequate processing may decrease the yield.
In an effort to overcome some of these difficulties, in recent years cellulose degradation methods using enzymes such as pectin hydrolases, cellulose, alkalis, or acids have been taught. In addition, the prior art teaches a number of processes for production of glucose from cellulose in the presence of lignin. Crushing and extraction processes for hydrocarbon-containing plants have also been taught. However, prior art processes have not dealt with the problem of obtaining hydrocarbons from hydrocarbon-containing plants wherein the hydrocarbon content is low and is contained in laticifer cells.
Additionally, traditional extraction methods make it difficult and inefficient to extract resins from plant materials, particularly from the bagasse. Bagasse is difficult to extract with hydrocarbon solvents for several reasons. First, the compounds of interest are adhered in the botanical matrix, so the material needs to be ground finely for accessibility of the solvent to these compounds. Second, the compounds of interest are significantly different in polarity, namely, resins are polar and rubber is non-polar. This makes it difficult to utilize a single solvent system, and therefore, most extraction processes utilize a two-solvent extraction system, e.g., acetone for resin extraction followed by cyclohexane for rubber extraction. Third, ground bagasse has physical properties that translate into very slow percolation rate for liquid solvents. Fourth, contact with oxygen can oxidize the rubber extract in other processes.
Thus, it has been difficult to design a commercially viable process for the extraction of bagasse with liquid solvents. Additionally, due to the problems with slow percolation rate through the bagasse, traditional processing methods have resulted in a low commercial output, and much of the unused bagasse contains residual solvents. The residual solvents in the remaining bagasse pose environmental safety hazards and make the excess bagasse mostly unusable for other applications. Finally, the low output makes these prior art extraction processes not commercially viable methods of extraction.
Therefore, a need exists for a cost-effective, efficient, and environmentally friendly method of extracting and fractionating rubber and resins from plant materials, such as guayule.